Determination of heat transfer intensity between free streaming water film and rigid surface using thermography

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1 Determination of eat transfer intensity between free ing water film and rigid surfae using termograpy Faulty of meanial engineering and naval ariteture University of Zagreb, Croatia Abstrat by S. Švaić, I. Boras and M. Suša Te goal of te resear was to find te relation for determining te eat transfer oeffiient between a free ing water film and metal orrugated surfae. Te relation between relevant parameters wi determine te eat transfer oeffiient as been obtained by means of Wilson-plot metod using data from te measurement. 1. Introdution Te open plate eat exanger (OPHE) onsists of two plates welded togeter so tat one is flowing between plates and anoter one over te outer surfae of upper plate. Te aim of su eat exanger design is to enable te eat exange between outer wi ould be waste dirty water and inner. Te OPHE are often used for eat reovery purposes in laundries, paper industries et. Te main problem in termo dynamial alulation of su type of eat exanger is determination of te eat transfer oeffiient on outer side. Beause te plates are always inlined to te orizontal plane and outer surfae is orrugated to ensure better eat transfer te standard relations for flat plate ould not be used. Te paper presents te results of te resear done on te one plate of OPHE wit te aim to find te relation for eat transfer oeffiient between ing film and plate. Te IR termograpy was used for measurements of temperature distribution on water film ing over te outer plate surfae. 2. Te problem Te open plate eat exanger is aimed for eat exange between ot waste water and old fres water. Waste water flows over te outer (upper) side of te OPHE plate in free and fres water flows inside, between te plates. OPHE are used in te ase wen waste water ontents partiles or oagulants. Te advantage of su onstrution is te possibility of easy leaning of te plates and module design wi enables enlargement of te unit. Te upper side of OPHE plate is orrugated to improve te eat exange between free of waste water and plate. Te OPHE plates are inlined (5 o ) to orizontal plane to ensure te free ing by means of gravity. Wile te equations for alulating te eat transfer oeffiient inside te plates ould be find in literature te relation for alulation te eat transfer oeffiient on outer side beame a problem. Te goal of te resear was to find te relation for determination te eat transfer oeffiient on te outer side of te OPHE plate. Te single plate of OPHE used in experiment is sown on figure 1 and te detail of orrugated upper surfae on figure 2.

2 Fig. 1. Te plate of eat exanger Fig. 2. Te features of upper plate 3. Experimental rig Te experimental rig onsists of single OPHE plate plaed on frame wit upper and lower ontainer (see figure 3). Te ot waste water enters te upper ontainer and overflow on te plate. After passing te plate te water enters te lower ontainer. Te fres water s in ounter flow between te plates.

Fig. 3. Experimental rig Te mass flow of waste (ot) and fres (old) water are measured by weiging wile te temperatures are measured by termoouples (see figure 4).

Hot inlet Hot outlet Cold outlet Cold inlet Fig. 4. Positions of termoouples and diretions of ot and old water 4.

3 Fig. 3. Experimental rig Te mass flow of waste (ot) and fres (old) water are measured by weiging wile te temperatures are measured by termoouples (see figure 4). Beside te termoouples te surfae temperature distribution of ot water ing over te plate was reorded by IR amera. All measurements were done in stationary state. Hot inlet Hot outlet Cold outlet Cold inlet Fig. 4. Positions of termoouples and diretions of ot and old water 4. Measurements Te measurements were arried out wit mass flow of 0,1; 0,2; 0,3 and 0,4 kg/s for ot and 0,1 and 0,2 kg/s for old. Some of te results are presented in table 1 and te related termograms are sown on figure 5.

4 Table 1. Te results of measurement using termoouples Measurement No. Hot Cold Mass flow [kg/min] 12,58 6,31 Inlet temperature [ C] 25,67 12,57 Outlet temperature [ C] 19,59 21,93 Mass flow [kg/min] 5,03 12,48 Inlet temperature [ C] 43,54 13,47 Outlet temperature [ C] 15,31 23,65 Mass flow [kg/min] 12 12,14 Inlet temperature [ C] 27,29 13,72 Outlet temperature [ C] 18,23 21,21 Mass flow [kg/min] 24,5 12,25 Inlet temperature [ C] 21,90 15,11 Outlet temperature [ C] 19,07 19,61 a) Measurement No.1

5 b) Measurement No. 2 ) Measurement No.3 d) Measurement No.4 Fig. 5. Termograms and referent temperatures

6 Te eat exanged between te s was alulated using relations: - for ot Φ = q ϑ ϑ kw (1) ( ) i o - for old Φ = q ϑ ϑ kw (2) ( ) o i Measurement No. Table 2. Te omparison of measurement's results Φ Φ termoouples Φ IR kw kw kw 1 4,119 5,327 4,55 2 8,861 9,883 8,59 3 6,343 7,580 6,68 4 3,845 4,843 4,09 It an be seen tat, using reordings by infrared amera, better auray was obtained for ot water in omparison wit tose obtained by termoouples. 5. Results By means of overall eat transfer oeffiients, alulated from measurement, and eat transfer oeffiient for old using Eq. (3), te eat transfer oeffiient for ot as been determined using Eq. (4). 0,8 0,4 λ = 0,022 Re Pr (3) d wall k λwall ekv 1 1 δ 1 = (4) Using Wilson-plot metod te relation Eq. (5) for determination of eat transfer oeffiient of outer an be found using data from measurements: 0,74 1/ 3 λ = 0,524 Re Pr (5) L Calulating te eat transfer oeffiient using Eq. (5) for ot and omparing tem wit tose obtained from measurement it an be seen tat good mutual orrespondene is obtained (see table 3). Table 3. Te omparison of eat transfer oeffiients for ot ; measurement and Eq.(5) Measurement No. from measurement alulated Eq.(5) W/m 2 K W/m 2 K ,7 2866, ,4 2286, ,5 2832, ,8 3376,5

7 6. Conlusion In te paper te appliation of IR termograpy in determination of eat transfer intensity as been desribed. Te speifi profile of eat exanger's surfae required an investigation for new relations for alulation of eat transfer oeffiients. Te results obtained using Eq.(5) for alulation eat transfer oeffiients on ot water side indiated a good orrespodene wit te measurement's result. REFERENCES [1] D.P.ALMOND, P.M.PATEL, Pototermal Siene and Tenique, London, Capman & Hall, [2] S.V. PATANKAR, Numerial Heat Transfer and Fluid Flow, Hemisfere Publising Corporation, M Grow-Hill Book Co, Wasington, [3] Gerald C. Holst, Common sense approa to termal imaging, SPIE Optial Engeenering Press, Bellingam, Wasington, USA, 2000

Determination of heat transfer intensity between free streaming water film and rigid surface using thermography

Determination of heat transfer intensity between free streaming water film and rigid surface using thermography IV Conferencia Panamericana de END Buenos Aires Octubre 2007 Determination of eat transfer intensity between free ing water film and rigid surface using termograpy Ivanka Boras and Srecko Svaic Faculty